It is called cell transport to the exchange of substances between the cell interior and the external environment in which it is located. This occurs through the plasma membrane, which is a semipermeable barrier that delimits the cell.
Cellular transport is vital for the entry of nutrients and dissolved substances into the environment, and the expulsion of residues or metabolized substances inside the cell, such as hormones or enzymes. According to its direction of movement of matter and its energy cost, we will talk about:
- Passive transport. By favoring the concentration gradient, that is, from a more concentrated medium to one less, it occurs by diffusion through the membrane and has no energy cost, since it takes advantage of the random movements of the molecules (their kinetic energy ). There are four types of passive transport:
- Simple diffusion. The material goes from the most concentrated to the least concentrated area until the levels are equal.
- Dissemination facilitated. Transport is carried out by special transport proteins found inside the cell membrane.
- Filtration. The plasma membrane has pores through which material of a certain specific size can leak into it, by hydrostatic pressure.
- Osmosis . Similar to simple diffusion, it depends on the passage of water molecules through the membrane, due to the pressure of the medium itself and its selectivity.
- Active transport. Unlike the passive, it works against the concentration gradient (from a less concentrated area to a more concentrated one), so it has a cost of cellular energy. This allows cells to accumulate the material they need for their synthesis processes.
Passive transport examples
- Dissolution in the phospholipid layer. Thus, numerous elements enter the cell, such as water, oxygen, carbon dioxide, fat-soluble vitamins, steroids, glycerin, and low molecular weight alcohols.
- Entrance for integral protein channels. Some ionic substances (with an electric charge), such as sodium, potassium, calcium, or bicarbonate, pass through the membrane guided by channels and special proteins for this, of very small size.
- The renal glomeruli. They filter the blood in the kidneys, stripping it of urea, creatinine, and salts, through an ultrafiltration process carried out by the capillaries, preventing the passage of larger elements and excreting the smallest ones thanks to the medium’s own pressure.
- Glucose absorption. The cells are always maintained with a low concentration of glucose, causing it to always flow by diffusion into it. To do this, transport proteins carry it inwards and then turn it glucose-6-phosphate.
- The action of insulin . This hormone secreted by the pancreas enhances the diffusion of blood glucose into the cells, reducing the presence of blood sugar, fulfilling a thermoregulatory role.
- Gas diffusion. Simple diffusion allows the entry of gases from respiration, from the exterior to the interior of the cells from their concentration in blood. In this way, CO 2 is expelled and oxygen is used.
- Sweating. The excretion of sweat through the skin is carried out by osmosis: the liquid flows out and carries toxins and other substances with it.
- The roots of the plants. They have selective membranes that allow water and other minerals to enter the plant, and then send it to the leaves to photosynthesize.
- Intestinal absorption. The intestine’s epithelial cells absorb water and other nutrients from the stool, without allowing it to enter the bloodstream. Said selectivity also occurs passively, through the electrolytic gradient.
- The release of enzymes and hormones into the bloodstream. It is often produced by the mechanics of high intracellular concentration, with no cost of ATP.
Examples of active transport
- Sodium-potassium pump. It is a mechanism of the cell membrane that allows, through a transporter protein, to expel sodium from inside the cell and replace it with potassium, maintaining ion gradients (low sodium and abundant potassium) and convenient electrical polarity.
- Calcium pump. Another transport protein present in the cell membrane, allows calcium to be carried against its electrochemical gradient, from the cytoplasm to the exterior.
- Phagocytosis. The white blood cells that allow us to defend the organism incorporates, through sacks of its plasma membrane, the foreign particles that we will later expel.
- Pinocytosis. Another phagocytic process proceeds through invaginations in the membrane that allow the entry of environmental liquid. It is something that the egg does during its maturation.
- Exocytosis. Contrary to phagocytization, it expels elements of the cellular content through membranous sacs that move outward, until they fuse with the membrane and open to the outside. This is how neurons communicate: transmitting ionic contents.
- HIV infection. The AIDS virus enters cells by taking advantage of their membrane, binding to glycoproteins in their outer layer (CD4 receptors), and actively penetrating inside them.
- Transcytosis. A mixture of endocytosis and exocytosis, allows the transport of substances from one medium to another, for example, from the blood capillaries to the surrounding tissues.
- Sugar phosphotransferase. A typical process for certain bacteria such as coli , which consists of chemically modifying the substrates inside them to attract others by covalent bonding and thus save a great deal of energy.
- Iron catchment. Many bacteria capture iron by secreting siderophores such as enterobactin, which binds to iron to form chelates and is then affinity-absorbed into the bacteria, where the metal is released.
- LDL uptake. This lipoprotein with cholesterol esters is captured cellularly thanks to the action of an apoprotein (B-100) that allows its entry into the membrane and subsequent decomposition into amino acids.